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Think of any non-opaque layer as if it were a filter through which you view the layers below. With only layers A and B visible, you're looking through Layer B at layer A, which has an effect similar to looking at Layer A through a filter that only passes half of the non-red light. (Provided, of course, that Layer A is white. The maths actually used only let this analogy go so far.) Turning on the visibility of Layer C means that you filter out half of the remaining non-red light. It's kind of like Zeno's Paradox—you can only ever get halfway there (although, since the numbers are integers, you'll eventually hit a practical 100% if you stack enough layers).

What you need, then, is a way to modulate the opacity of the filtering layer, and that's done with a layer mask (or alpha channel if you're working with an imported image). A mask fill at 50% grey with a layer opacity of 100% will give you the same effect as a layer with no mask and an opacity of 50%. The difference is that you can add lighter or darker areas to the mask (black blocks, white weeveals) to modulate the strength of the effect from 0 to 100%. Exactly how you'd go about that depends on what you're actually doing (which is not clear from the question as asked).